Electrical wave modulation



April 1 8, 1944.' E. l. GREENy 2,347,098

ELEQTRICAL WAVE MODULATION Filed Jan. s. 1942 F/a/ 2 3 ,[5 A Af.

CHANNELS l -IZ RE C.

/NVENTOR E. GREEN TTORNE Y Patented Apr. 18, 1944 vinurl-:o STATESPATENT oFFics aLEcrmo'An WAVE MoDULATIoN Estill I. Green, Millburn, N.J., assigner to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application January 9, 1942, Serial No.426,146

5 Claims. ('01. 179-1715) -high quality speech systems. measuringsystems The present invention relates to the production of modulatedelectrical waves for carrier wave signaling or other purposes.

Many ways of producing electrical wave mociulation have been devisedsince the advent of wireless telephony and wire carrier telephony, themost commonly used types of modulators helng those using some form ofvacuum tube circuit and those using solid element varistors such ascopper-oxide rectiiiers or blocks of composite material having anon-linear volt-ampere characteristic. To a lesser extent, magneticmcdulators have been used.

While these types have proved generally satisfactory for most uses, theyhave the drawback oi causing inteiniodulation products between allimpressed wave components and where the sianal itself has 'a number ofdiiierent frequency components oi comparable amplitude theintermodulation products between such components will in some uses provetroublesome. These have commonly been kept to ncgligibly low value byand similar situations.

For convenience oi* expression the constant frequency wave that is to bemodulated will be termed herein the carrier wave whether it he acatually used as the transmission carrier wave or merely for frequencyshifting purposes in a local circuit, as in a superheteiodyne system orheat wave system or certain types oi measuring circuits using afrequency shift.

A general object ofthe present invention is a modulating system ormethod oi such characterthat the intermodulation products between diiferent components of the signal or other mcd ulating wave areinherentlyj kept at a low substantially zero value.

This object is attained in accordance with the use of a large carrieramplitude relative to signal amplitude. 4

One use in which the intermodulaticn products between input waves in thesame input circuit are particularly troublesome is instanced in groupmodulators and demodulators, which are used for shitting anentire groupof modulated high relative to the other input waves since the.

modulated wave channels incoming to' the group modulator or demodulator'are often at fairly high level because' of transmission requirements'use of a modulator of the contact resistance type and varying itsresistance oy mechanically vary ing the contact pressure at the carrierfrequency.

The nature ofthe invention and its various features and objects will bemore fully understood from the following detailed description taken inconjunction with the accompanying drawing, in which:

Fig. l is, a circuit diagram in block schematic oi' one terminal of atwo-way carrier telegraph system incorporating group modulators inaccorglance with the present invention;

Fig. 2 is a simplified circuit diagram showing impedance relations to hereferred to in 'the description: and

Figs. 3, 4 and 5 are apparatus diagrams showing threedierent forms whichthe group mod 'ulator in accordance with the invention may placed on thesystem.4 In certain prior art systems the incoming channels have beenprovided with loss pads to lower their level to help bring about thehigh amplitude relation of the shifting wave with respect to the channelamplitudes.

Y This procedure requires ampliilcation after modulation to restore thechannel levels to desired value for transmission purposes. i

Other cases in which it is desired to keep the intermodulation productsat low level include take, these forms being applicable also to othertypes of modulation.

Referring to Fig. 1, the general plan of this circuit is old in theart..the feature of novelty inhering in the usein such circuit of thegroup modulators in accordance with the present invention,the nature o!these modulators to he referred to subsequently in the description. As

shown in the drawing, one group oi carrier telegraph channels comprisingchannels i to 12,. inclusive, transmit directly to the eastward line Il.Two of these channels are indicated in some suitable frequency such as425 cycles per second,

followed by a transmitting circuit 2 and a 425- cycle lter '3, thepass-band of which is sumciently wide to accomodate the essentialfrequencies of whichl the signal in this channel is composed. Thetransmitter 2 is indicated as under the control of a key d. The nexthigher vchannel is indicated as using a carrier frequency of 595 cycles.Thisv same frequency separation of 170 cycles between channels is usedthroughout the group so that channel No. 12 of this group uses afrequency of 2,295 cycles as indicated on the gure. These twelvechannels transmit out through the transmitting amplifier 5 and low-passfilter t to the outgoing line l il.

The second group of channels comprising channels-13 to 24, inclusive,may be entirely similar to channels Nos. l to 12 up to the point wherethe group modulator l2 is connected into the circuit. The purpose of thegroup modulator I2 is to raise the frequencies of these channels 13 to24,

-transmitted through the group modulator filter I5 while the oppositeside-band is suppressed. The selected side-band is amplied at I6 andtransmitted through high-pass lter il to the line i0.

'Ihe manner in which these channels terminate at the opposite terminalmay be seen from considering the west terminal for westward line 2li atthe lower part of the gure. incoming over line 2li are selectivelytransmitted through direction'alloW-pass lter 2i and receiving amplifier22 'to the input side of the twelve channel filters, one of which isshown at 23. These channel lters separate the message currents belongingto the individual channels and direct them into the various receivers,such as 2li, the outputs of which are connected to relays, such as 25,for receiving the message.

I'he upper group comprising channels 13 to 24 is selectively transmittedthrough the high group directional lter 2t and receiving amplier 21 andimpressed on group modulator 2B. This group modulator is also suppliedwith some of the group shifting frequency 4,936 cycles from the outputof lter it, by virtue of which these channels as a group are steppeddownward in vfrequency to occupy the same frequency position l as theyoriginally had before reaching the group modulator of the distanttransmitting station. From this point on the manner in which thesechannels are terminated is entirely similar to that described forreceiving channels 1 to 12.

Each of the transmitting wave sources l and the group shifting source I3may in practice be a separate source or all of these frequencies may beobtained from a multifrequency generator or by means of a harmonicgenerator from a single .base frequency of 85v cycles per secondfrequency.

Y of the type disclosed in a paper entitled Voice frequency carriertelegraph system for cables by Hamilton, NyquistfLong, and Phelps,published in the Journal of the American Institute of ElectricalEngineers, Volume XLIV, No. 3, March and described above, the twelvetransmittingl 'channels comprising channels 1 to 12, inclusive,

communicate directly over the line l0 with the corresponding receivingchannels at the distant station or stations by means of their individualcarrier frequencies and without the necessity of frequency translation.Channels 13 to 24 have their frequencies stepped up by the groupmodulator I2 at the transmitting point for transmission over the line I0to the distant terminal station or stations 'Where their frequencies arestepped downward as a group by a modulator corresponding to groupmodulator 28 for reception by the channel terminal apparatus. sired,other blocks or groups of channels may be similarly superposed i-n thefrequency spectrum and transmitted over the same line by use of similargroup modulators with appropriate shifting Channels 1 to 12 frequencies.It is important in this process of shifting the frequencies of thetwelve channels as a group that intermodulation between individualchannels be kept downto a minimum to avoid interference or crosstalk andreference will now be made to a type of modulator in accordance with thepresent invention which may be advantageously used as the groupmodulator in a system such as that shown in Fig. 1. It will beunderstood, of course, that the modulator of the invention is alsocapable of general application and that when used as a group modulatorthe actual system may diier widely from that shown in Fig. 1 which is tobe taken merely as illustrative of the use of the modulator of theinvention for group modulating purposes. v While the multiplex systemchosen for illustration is a carrier telegraph system, this is not to beconstrued as limiting the invention, which is applicable to multiplexcarrier telephony or other kinds of signal transmission systems. i

Referring to the elemental circuit diagram of Fig. 2, the modulators tobe described in accordance with the present invention comprise in eachcase a variable resistance `R. connected in series .with the source ofsignal' voltage E and with the load impedance Z. The instantaneous valueof the modulator resistance R is mechanically varied at the carrierfrequency of q radians-per second. Further reference to this Fig. 2 willbe made later on in the description in connection with the explanationof the theory of operatic involved.

One form which the group modulator of the invention may take isindicated in Fig. 3. It comprises a variable resistance composed ofgranular carbon 30 contained between an upper electrode 3| and a lowerelectrode 32. This variable resistance is s erially connected into thecircuit by means of terminaison the conductive membersl and 32. Thelower member 32 is mounted to be vibrated at the carrier frequency/,

by means of the piezo crystal 33 having upper and lower plates connectedto the source of carrier waves. The upper plate is shown mechanicallyconnectedby insulating layer 31 to the bottom conductive plate 32 of thecarbon,

chamber. By this means mechanical pressure is exerted on the carbongranules, varying at the carrier frequency and the instantaneous valueof the resistance offered, by triegranular carbon mass is thereforevaried at thecarrier frequency. This resistance varying in carrier2,347,098l frequency is serially included between the source.

of signals and the load circuit.

Fig. 4 is similar to Fig. 3 except that two such granular carbonreslstances 30 and 30 are con nected in push-pull relation and aremechanically driven by a common crystal 33.` Input transformer 34 andoutput transformer 35 with a mid-tap connection 36 leading from themidpoint of secondary windingv of transformer 36 to the mid-point of theprimary winding oftransformer 35 are provided for coupling the modulatorproper to the external circuit.

Referring to Fig. 5, another method of vibrating the granular carbonresistance 30 is shown comprising a magnetostriction member 40 providedwith a bias winding 4| and bias source 42 and a driving Winding 43supplied with current of the carrier frequency. Elongations andcontractions of the magnetic core 4i) at the carrier frequency impartvibrations cf corresponding frequency to the mass of carbon 3U. Forfurther information as to' how magnetos'triction may be employed forsuch purpose reference may be made to United States Patent 1,092,453 toP. M. Rainey, April 7, 1914.

Reference is made in connection with Fig.,4

to input and output terminals.4 It will be clear,

however, that the type of modulator shown in each of Figs. 3, 4 and 5 isbilateral so that either par of signal terminals shown may be input oroutput terminals.

A consideration of the theory of operation will serve to demonstratethat theoretically no crossmodulation products between components of thesignal are generated in the contact resistance modulator of theinvention. The well-known vacuum tube modulator is operated on a curvedpart of its characteristic, which means that if the carrier voltage isremoved and4 only the signal (such as speech) is applied, an outputconsisting of the original speech, reproduced and possiblyamplifled. isobtained along with lntermodulation products between the differentspeechcompcnents since the input-output characteristic is assumed to benon-linear, the tube being operated as a modulator. If this simple :testis applied to magnetic, capacity or recti-v fier type modulators, itwill be found that they product at least some degree of .intermodulationbetween signal components for the same reason thatl they are able to`modulate a carrier wave bythe signal, namely they operate with anon-linear input-output characteristic. In the case of the contactresistance modulator of the invention, if the carrier wave (in thislcase the source of mechanical vibration) be kept zero and the signalalone applied, no intermodulation between signal 'components appearssince the modulator acts simply as an ohmic resistance of fixed value.

This may also be seen to be `the case by inspection of the formulaeapplicable to this type of circuit. Referring to Fig. 2, we can write,

R=To+r sin qt `beingq (inradianspersecond).

CII

The signal input'comprising several different frequencies may bedesignated as Ezar sin pli-l-az sin pzt ai and az being coefficients andp1 and pg being frequencies (in radians per second).

The current flowing in the circuit is E la' V:al sin pit-tag sin p2! Z-l-lt Z-i-rO-l-r sin qt Calling the denominator constants Zi this expandsto It can be seen from inspection that when this expression ismultiplied out,l it yields no term containing a product of sin pit timessin pet, which indicates that no intermodulation products between theseparate signal components occur.

- What' is claimedgis:

1. A modulating system comprising a source of signal frequencyelectrical waves, a load cir-1 cuit and in series therewith a resistancecontact whose resistance value is variable with variation of contactpressure, and means to vary the contact pressure at the frequency of thewave to be modulated.

2. Ir'i a modulating system, a source of modulating electrical wavescomprising a band of frequencies, means for modulating a carrierfrequency by said waves simultaneously and individually lcomprising acontact resistance, and means for mechanically varying the contactpressure at the carrier frequency.

3. In combination, a source of electrical waves covering a band offrequencies, a contact resistance and a load in series with said source,means to vary the contact pressure at a constant frequency outside saidband, and means comprised in said load for responding to the modulatedwaves resulting from such variations in4 contact pressure.

of different frequencies comprised within a given frequency band, a.contact resistance and load in series with each other and said circuit,and

means to shift the frequencies of said pluralityv of signal waves to adifferent frequency level comprising means to vary the oontact'pressureof ce where ro is the constant component of the re said contactresistance at a frequency outside said band.'

5. In a modulator, a source of mechanical vibrations of carrierfrequency, a contact resistance mounted to have its contact pressure andtherefore its electrical resistance varied at carrier frequency by saidmechanical vibrations, a`

source vof modulating electrical waves differing infrequency from saidcarrier frequency, a uti.- lization circuit, and means connecting saidutilisation circuit, said contact resistance and said source ofelectrical waves in series with eachl other.

EBTILL I. GREEN.

